Mars Global Surveyor
Mars Orbiter Camera (MOC)

Mars: An Active Planet

MGS MOC Releases MOC2-166 to MOC2-172, 10 August 1999

Among the goals of the Mars Surveyor program are to characterize
the planet's climate and the interaction of the atmosphere with
the planet's surface. Both the Mars Global Surveyor (MGS, presently
in Mars orbit) and the Mars Climate Orbiter (MCO, which reaches Mars
in September) address these goals in part by using cameras to
observe martian weather and changes on the surface that occur from
season to season.

Mars Global Surveyor has been orbiting the Red Planet for just over 1
martian year (687 days). Although the spacecraft only recently attained
its final design orbit after 18 months of aerobraking and other orbit
phasing activities, "snapshots" of Mars acquired during this period by the
MGS Mars Orbiter Camera (MOC)--when it has been on--and more recent views
from the present mapping orbit have captured a unique record of seasonal
and meteorological events that demonstrate that the planet is quite
active and dynamic today.

Evidence for present-day activity comes in two forms--weather, and
surface changes. Detailed weather observations include the tracking
of dust devils and the daily mapping of cloud and storm patterns.
Other changes on the planet have been seen among frost-covered sand dunes.
These changes are connected to the passage of martian seasons and the retreat
of polar ice as winter draws to a close and spring begins. As the
winter ice begins to sublime, dunes develop small dark spots
that grow and eventually coalesce until the frost disappears. Some
dunes show evidence that wind and gravity are actively moving the dune
sands, as well.

The images shown below were presented at a Space Science Update
briefing at NASA Headquarters, Washington, DC, on 10 August 1999. For
the corresponding JPL Press Release, see:
www.jpl.nasa.gov/news/.

1. The Mystery of the Spotted Dunes

MOC2-166
"Bushes" On Dunes
August 1999

MOC2-167
Snow Leopard Dunes
August 1999

MOC2-168
Changes In 26 Days
August 1999

MOC2-169
Wind Streak Dunes
August 1999

Every year, Mars grows two large, seasonal frost caps (one during each
hemisphere's winter) out of a combination of atmospheric carbon
dioxide (the major component of the cap) and water vapor. At the end
of each winter, these caps recede in latitude as the sun moves across
the equator and into the spring-time hemisphere. This annual cycle of
frost deposition and sublimation (i.e.,the process by
which ice transforms when heated directly from solid to gas without
first "melting" to form a liquid) is one of the definining elements of
the martian climate. There is much that is unknown about this
process, but recent observations of sand dunes within the polar
regions are providing new information about the seasonal retreat of
the polar ice caps.

Observations made in 1998 at the end of the north polar winter showed some
evidence that dark spots develop on sand dunes as the winter frost begins to
dissappear. This year, 1999, similar features have been observed in the
southern hemisphere as winter has proceeded into spring (southern spring began
August 2, 1999). The evidence from the dunes suggests that defrosting
begins more or less simultaneously at many small, localized sites. Each site
then grows radially from the initial spot, enlarging and eventually
merging until all of the frost is gone. The rate of growth of the defrosted
surfaces, and the details of their appearance, indicate that the frost is
probably repeatedly sublimed and locally re-deposited, with this local
cycle acting to "refridgerate" the ground and moderate or slow the rate
of polar cap retreat. Each picture shown above (MOC2-166 to MOC2-169)
provides examples of the dark spots that
develop on dunes as they "thaw out," including comparisons over a 26-day
period and an example of local wind transport of sand exposed in the spots.

2. Recent Dune Activity

MOC2-170
Proctor Dunes
August 1999

Since first seen in Mariner 9 images of Mars, the isolated dune fields within
large impact craters have been of great interest, as their dark color
indicates that the light dust that covers much of the planet does not
accumulate on the sandy surfaces. This indicates that the dunes must
be active--moving--and that we might, with time, eventually see evidence of
changes that allow us to measure the effectiveness of wind erosion on
Mars. The dune field in the picture above shows evidence of recent
activity, as dark sand has been mobilized and transported across surfaces
covered by the late-winter remains of seasonal frost.

3. Dust Devils

MOC2-171
Dust Devils
August 1999

Dust devils result from spinning vortices of air that lift dust from
a planet's surface. They occur on Earth, where they are relatively
small features and are commonly seen on hot, dry summer days in desert
and farmland settings. On Mars, dust devils are thought by some to
be a major source of the fine, pinkish dust that gives the sky
its unearthly brownish color (as seen from the Mars Pathfinder
and Viking landers). The MGS MOC wide angle camera has been used
to look for dust devils--a clear demonstration that the planet
is active today.

4. North Polar Clouds

MOC2-172
North Polar Weather
August 1999

The MGS MOC wide angle cameras were designed specifically to monitor
and document the red planet's weather over the course of a martian
year. At the north pole, storm clouds have been brewing all
through July and August 1999, as northern summer transitioned into
fall. The picture shown here above is one of a series that shows how the
weather patterns on Mars evolve over a couple of days.